Sudden cardiac death (SCD) accounts for 300,000-400,000 deaths per year in the United States. Although the individual risk of SCD in the adult U.S. population is only about 0.1-0.2% per year, when applied to the large population base of the U.S., SCD is often the first and only manifestation of cardiovascular disease in a majority of cardiovascular related deaths. Deaths from patients recovering from large myocardial infarctions actually represent the minority of the total deaths per year. As a result, a low cost screening tool that would provide early detection of patients at risk for SCD would be tremendously valuable for early treatment and intervention.
However, it can be difficult to accurately predict or assess the risk of SCD because many underlying pathologies support or trigger the events leading to SCD instead of any single condition. Of these various conditions, most data suggests that autonomic regulation of the heart through the branches of the sympathetic and vagal systems is extremely important in maintaining stable rhythms. In particular, it appears that vagal stimulation mitigates the development of ventricular arrhythmias in a variety of experimental studies. A strong vagal response appears to lower heart rate and lower the risk of arrhythmia, which effectively counteracts reflex sympathetic hyperactivity and provides a key determinant for survival.
The dual branches of the autonomic system regulate heart rate over the course of an exercise test. Increasing exertion during exercise increases perfusion demands and results in a higher heart rate. The higher heart rate is caused by increased activity of the sympathetic system and decreased parasympathetic regulation. During an initial stage of recovery after terminating exercise, sympathetic stimulation is withdrawn. After the initial stage of recovery, vagal mediation necessary to return the heart rate to a resting value is reasserted to further reduce the heart rate. Heart rate recovery parameters, measured as the difference between the peak heart rate and the heart rate at subsequent times during recovery (e.g., 1 or 2 minutes into recovery), provide a gross quantification of the combined effects on heart rate from the activities of both sympathetic and vagal branches of the autonomic system. Numerous studies have examined the prognostic value of heart rate recovery parameters for assessing patient risk based on the vagal contribution to the recovery and the relationship between poor vagal tone and cardiovascular-related mortality. Even when key co-morbidities were quantified, such as perfusion defects and coronary artery disease (CAD), the heart rate recovery has been found to be a significant and independent predictor of all-cause death.
Reassertion of vagal tone (i.e., vagal mediation) following exercise is an important component in the heart rate recovery process. It has been found that patients with poor vagal tone are at increased risk of cardiovascular mortality, and many heart rate recovery studies have attempted to accurately quantify the vagal tone during recovery. However, it is not clear that traditional measures of evaluating the heart rate recovery accurately quantify the vagal tone. More specifically, traditional measures of evaluating the heart rate recovery of a specific patient may be adversely influenced by the sympathetic withdrawl. Heart rate recovery is also strongly correlated with, and becomes identical to, the heart rate reserve as recovery time increases. The heart rate reserve in such traditional studies is the peak heart rate less a measured pre-exercise supine resting heart rate. Additionally, normalizing a heart rate recovery value of traditional techniques by the heart rate reserve and assessing the result as a percentage of total recovery renders the index non-prognostic. This raises further concerns that traditional techniques for evaluating the heart rate recovery of a patient may have questionable value in characterizing vagal tone.
Although the traditional methods for measuring heart rate recovery are well known to practitioners of the art, and important observations have been made in many previous studies, the works of Cardiac Science, Inc. and others suggest that such traditional methods for evaluating heart rate recovery result in a disguised metric for peak heart rate or heart rate reserve. As a result, traditional methods for evaluating heart rate recovery fail to accurately quantify the vagal tone of a specific patient. Therefore, it would be beneficial to extract vagal tone information in a way that is both prognostic and independent of peak heart rate and traditional heart rate reserve parameters to provide a new and accurate parameter for patient risk stratification.